The present invention relates to a coal fired steam generating system and method which produces low emissions of nitrogen oxides, employs low excess air and maximizes overall efficiency.
Nitrogen monoxide (NO) and nitrogen dioxide (NO.sub.2) are byproducts of the combustion process of virtually all fossil fuels. Historically, the quantity of these inorganic compounds in the products of combustion was not sufficient to affect boiler performance and their presence was largely ignored. In recent years, oxides of nitrogen have been shown to be key constituents in the complex photochemical oxidant reaction with sunlight to form smog. Today, the emission of NO.sub.2 and NO (collectively referred to as NO.sub.x) is regulated by both state and federal authorities and has become an important consideration in the design of fuel firing equipment.
The formation of NO.sub.x in the combustion process is often explained in terms of the source of nitrogen required for the reaction. The NO.sub.x can originate from the oxidation of nitrogen in atmospheric air in which the product is referred to as "thermal NO.sub.x " or from the organically bound nitrogen components found in all solid and liquid fossil fuels which are termed "fuel NO.sub.x ". The formation of thermal NO.sub.x can be decreased by reducing the residence time, the combustion temperature, and the concentration of O.sub.2. On the other hand, the fuel NO.sub.x is less temperature dependant, but is a strong function of the fuel-air stoichiometry and residence time. A number of techniques to control fuel NO.sub.x have been developed that involve modification of the combustion process such as low excess air firing and air staging. Under fuel-rich conditions and with sufficient residence time available, the conversion of fuel nitrogen to harmless molecular nitrogen, rather than to NO.sub.x can be maximized.
One technique for reducing the formation of NO.sub.x is the use of air staging or overfire air by which the combustion process is spread out. The overfire air nozzles are located in the windbox above the uppermost coal nozzles. Approximately 20% of the total combustion air to a burning zone is introduced through these overfire air nozzles. As a result, the fireball is at slightly sub-stoichiometric air conditions. When combined with low excess air firing in the range of perhaps 15 to 20% excess air, the NO.sub.x formation is controlled by driving the major fraction of the fuel nitrogen compounds into the gas phase under overall fuel-rich conditions. In this atmosphere of oxygen deficiency, there occurs a maximum rate of decay of the evolved intermediate nitrogen compounds to N.sub.2. Following the introduction of the remaining overfire air, the slow burning rate reduces the peak flame temperature to curtail the thermal NO.sub.x production in the later stages of combustion. The use of even lower levels of excess air (below 15%) would further reduce the formation of NO.sub.x and increase plant efficiency but that normally results in the incomplete combustion of the fuel and high levels of unburned carbon in the flyash thereby reducing efficiency.
Another one of the developments that has been used to reduce the formation of NO.sub.x is the offset air or concentric firing technique disclosed in U.S. Pat. No. 4,294, 178. In this firing technique, tangential firing is employed with the fuel and primary combustion air being introduced tangentially to an imaginary circle in the center of the furnace and with the secondary combustion air being directed tangentially to a larger concentric circle. This patent also discloses the use of flue gas recirculation which is also tangentially introduced between the fuel and secondary air streams. This concentric or offset air firing technique is another way of spreading out the combustion process and has the effect of reducing the formation of NO.sub.x. Simultaneously, it also reduces the slagging and corrosion of the furnace walls. With deep staged firing where conditions at the burner level would normally be a reducing atmosphere, severe slagging and corrosion of the water walls would occur. Therefore, with this deep staging, it is necessary to use concentric firing where the secondary combustion air is on the outside of the fireball against the walls, so that an oxidizing atmosphere is maintained at the wall. This reduces slagging, corrosion and the need for soot blowing.